I wouldn't worry about it. If your beer tastes great, just enjoy it and worry about getting good readings the next time. Just remember to spin the hydrometer when you put it in the sample tube to dislodge any bubbles, then make sure it's not touching the sides. Finally, many hydrometers are off. Because you're using one hydrometer to measure such a wide range they can be off on the low or high side. I noticed I could never hit my preboil gravities and always seemed to have poor efficiency. I tested my hydrometer in some 60 degree water and it was off by 4 points!